1. Field of the Invention
This invention relates generally to wireless communications, and more particularly to ultra-wide band communication methods and systems.
2. Description of the Prior Art
Ultra Wide Band (UWB) communication systems promise the maximum data rate for dc power expended. These systems are characterized by microwatts of transmitted power, bandwidths of ten times the data rate, digital coding for processing gains of 100 and center frequencies nearly the same as the bandwidth. Known UWB communication systems generally utilize wavelet waveforms having a bandwidth of several GHz.
Present low-power (FCC Part 15, ISM bands) unlicensed, undefined modulation systems can transmit at RF power levels up to 0.7 mw, equivalent to 50 mv/m at a distance of 3 meters. This implies, for example, a transmit power level of xe2x88x921.25 dBm for a dipole antenna (isotropic). In contrast, intentional radiation RF power levels outside of the ISM bands is limited above 960 MHz to 500 uv/m (xe2x88x9221.25 dBm).
Because of their very small level of transmitted power, UWB systems utilize the VHF and UHF bands for optimal in-building communication. Unfortunately, above 960 MHz, there are two bands which are extremely sensitive to interference. These are the 1220 MHz and 1575 MHz global positioning system (GPS) satellite link communication frequencies.
As UWB communication systems continue to develop, it is foreseeable that new applications may necessitate consideration of intentional radiation levels below the present class B unintentional level of 75 microwatts per GHz of bandwidth. These applications, however, may conflict with the needs of present xe2x80x9cownersxe2x80x9d of commonly used bandwidth, even when associated radiation levels are limited in range to 10-50 meters. Present xe2x80x9cownersxe2x80x9d may include, for example, cellular phone system operators communicating at frequencies such as 825-869 MHz and 1900 MHz and global positioning system (GPS) operators communicating at frequencies such as 1225 MHz and 1575 MHz. Therefore, a need presently exists for a low-power UWB communication system that can be operated in a manner that is capable of excising the potential transmitted interference among simultaneous communication system operators.
Present techniques for removing transmitted signals in the foregoing communication bands are limited to physically notching the transmit antenna to minimize its power at the frequencies to be excised and/or inserting RF notch filters between the transmitter and the transmit antenna. These techniques are somewhat limited in that notches presently can only be applied to laboratory prototype transmit antennas at lower center frequencies. Further, use of notches does not completely suppress the waveform, but simply changes the radiation characteristics, for example, from directional to omni-directional. Although precise, narrow band RF notch filters are undesirably large and lossy.
In view of the foregoing discussion, a need exists in the wireless communications art for a low-cost, low-power, highly efficient UWB communication system capable of selectively removing transmissions, without use of notch filters or frequency shifting techniques, in designated bands that are narrow relative to the full bandwidth being utilized by the UWB communication system. Such a communication system would be very advantageous since it would be capable of short range, indoor communication without concern for signal losses associated with objects, e.g. walls, doors, windows, appliances, or other structures, due to reflections and the like, when operating within or near the broadcast frequency band (i.e., about 100-200 MHz). Such a communication system would thus be able to take advantage of the xe2x80x9csweet spotxe2x80x9d range of frequencies (about 100-200 MHz) normally associated with indoor, battery operated devices.
The present invention is directed to a low-cost, low-power, highly efficient ultra-wide band (UWB) communication system capable of selectively removing transmissions, without use of frequency notch filters or frequency shifting techniques, in designated bands that are narrow relative to the full bandwidth being utilized by the UWB communication system. One preferred embodiment of the present invention comprises an expander or spreader to process an incoming stream of data in the frequency domain into a waveform that is a function of time. One embodiment implements this expansion process by multiplying, via a mixer, the incoming stream of data with a chirp waveform from a chirp generator, thereby creating an output waveform in which each frequency is associated with a specific unit of time. Most preferably, each frequency will be associated with no more than a single unit of time. The processed waveform that is a function of time is then passed through a time notching process (as contrasted with known frequency notching techniques discussed above) in the time domain by synchronizing the processed waveform with a series of notches implemented in the time domain. In this manner, it is relatively easy to generate the time domain pulses necessary to very accurately excise any transmission frequencies desired to be excised from the transmitted signal. This technique is especially useful where the frequency bands to be excised are narrow relative to the full transmission bandwidth.
The present invention provides various technical advantages. In one aspect of the invention, a technique provides for a maximum limit on peak transmitter power such that a 100xc3x97 factor in range may be attained in a quiet (i.e. home) environment, thereby enabling a transmitter to back-off to the minimum RF power for a given bit error rate.
In another aspect of the invention, a technique provides an advantage when used in the same environment as unlicensed direct sequence (DS) radios. This technique provides a 20 dB processing gain (PG=100) with xe2x88x921.25 dBm peak power, giving an effective 19.75 dBm transmitted power. Present ISM band unlicensed DS radios are limited to 8 dBm with +10 dB gain, effectively providing 18 dBm transmitted power. The present invention therefore has a 1.75 dB advantage over unlicensed DS radios in the ISM bands.
In yet another aspect of the invention, a technique enables several users to operate in the same frequency range without frequency channelization using several expensive IF filters, e.g. placing resonators at respective shunt elements in a lattice structure in an attempt to create a narrow-band notch.
In still another aspect of the invention, a technique enables precise (xe2x80x9cbrick wallxe2x80x9d) notching of many forbidden frequency bands.
In another aspect of the invention, a technique provides precise notching in association with signal processing (DSP) at baseband.
In yet another aspect of the invention, a technique enables near perfect (steep, minimal transition) excision of selected transmission frequencies.
In still another aspect of the invention, a technique enables implementation of a low-cost, low-power UWB communication system using known building blocks, e.g. reasonably good RF transceiver that does not require a high performance local oscillator (phase noise relaxation); chirp modulation; and demodulation circuits to implement spread-spectrum, such as an oversampled digital correlator, or SAW correlator, that does not require synchronization.